--> Lateral Facies Trends in Deep-Marine Basin-Floor Matrix-Rich Sandstones, Neoproterozoic Windermere Supergroup, Western Canada

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Lateral Facies Trends in Deep-Marine Basin-Floor Matrix-Rich Sandstones, Neoproterozoic Windermere Supergroup, Western Canada

Abstract

Although first described in the 1960's, it is only recently that matrix-rich sandstones (MRS's) have been widely recognized in the ancient deep-marine sedimentary record. In spite of this, their depositional mechanisms remain a source of ongoing debate, in part because details of their lateral lithological changes are poorly known. Previous work in the Upper Kaza Group of the Neoproterozoic Windermere Supergroup turbidite system has shown that matrix-poor sandstones (MPS's) transition laterally to MRS's (30 to >50% matrix-content), and then into thin-bedded turbidites (TBT's), but the transitions were typically poorly exposed. This study investigates these transitions in a well-exposed, few meter thick by ~650 m wide lateral transect through proximal basin floor deposits. In the study area, MPS beds (10–15% matrix-content) are ~50 cm thick and consist of scour- or load-based, massive to normally-graded coarse-grained sandstone with abundant granules. Over 60 m laterally, matrix-content increases to 20–30%, grain size decreases to medium sand, mud clasts increase from <1% to 5–10% of bed volume, and beds gain ~3 cm silt/mud caps. In the next 45 m, matrix-content increases gradually to 30–40%, beds thin to ~20 cm, larger mud clasts comprise 15% of bed volume, and granules are rare. At this point, beds are massive to normally-graded, planar-based MRS's that progressively, over 114 m, transition to ~10 cm thick, fine-grained sandstone with 40 to >50% matrix-content, more stratified silt/mud caps (couplets of ~0.8 cm ripple cross-stratified fine sand overlain by ~0.5 cm silt/mud), and smaller wispy mud clasts. Over the next 285 m, MRS beds thin to ~1 cm, matrix-content decreases abruptly to 20–30%, and mud clasts are absent. Additionally, the caps of MRS beds gradually thicken and become TBT's, which generally consist of Tcd successions with ~0.2–2.5 cm fine sand Tc's and ~2 cm silty mud Td's. Here, MRS beds can be traced directly into Tc ripple cross-stratified fine sandstone. The observed lateral facies changes are interpreted to be the result of rheological changes within a succession of high-energy plane-wall jet flows that locally had deeply eroded a mud-rich sea floor. Changes were most profound along the comparatively lower-energy margins of the flows, and it is here that MRS's form a unit ~160 m wide, which thereafter transitions rapidly into upper division turbidites deposited from the now fully diluted, and hence low energy, turbidity currents.